High pressure electric discharge lamp and electrode therefor

ABSTRACT

A high pressure electric discharge lamp has a fused silica arc tube having press seals disposed at each end thereof with molybdenum ribbons embedded within the press seals. Disposed within the arc tube, at the ends thereof, are electrodes which are connected to the molybdenum ribbons. The electrodes are made of refractory metal powder, pressed and sintered into a tubular shape. A charge of emitter material is disposed and pressed within an axial cavity in the electrode against a refractory metal rod at the rearward end of the electrode.

' United States Patent 1191 Pollard Oct. 28, 1975 HIGH PRESSURE ELECTRIC DISCHARGE 2,8 7,603 5/1959 l-laidinger 313/184 AM AND ELECTRODE THEREFOR 3,405,303 10/1968 Koury et al...... 313/217 3,548,242 12/1970 Ayotte et al. 313/184 Inventor: Stuart Pollard, Greenland. NH 3,700,951 10/1972 Clarke et a1. 313/184 x [73] Assignee: GTE Sylvania Incorporated, Danvers, Mass 1 Primary Examiner-Palmer C. Demeo Attorney, Agent, or Firm.lam'es Theodosopoulos [22] Filed: Oct. 15, 1973 v [21] Appl.. No.: 406,566 [57] ABSTRACT Related Application Data 1 high pressure electric discharge lamp has a fused 63 C f S U N 26 silica arc tube havmg press seals disposed ateach end 1 8: 3:12? 0 er 0 2606 J 1972 thereof with molybdenum ribbons embedded within the press seals. Disposed within the arc tube, at the U 52 US. Cl. 313/184; 313/211- 313/217 ends there, are electwdes which are 51 1111. C1. 1101.! 61/073; H01J 61/12 the mdyhdenum i h Theelectmdhs are made 0f [58] Field of Search 313/184 211 217 whammy metal F Pressed and Wired a tubular shape. A charge of emitter material is disposed [56] References Cited and pressed within an axial cavity in the electrode UNITED STATES PATENTS, against a refractory metal rod at the rearward end of the electrode. 2,460,739 2/1949 Francis 312/211 x 2,821,647 1/1958 Rigden 313/184 2 s, 2 Drawing Figures U.S. Patent Oct. 28, 1975 3,916,241

FIG.2

HIGH PRESSURE ELECTRIC DISCHARGE LAMP AND ELECTRODE THEREFOR This is a continuation of application Ser. No. 262,606, June 14, 1972, and now abandoned.

BACKGROUND OF THE INVENTION 1. Field Of The Invention This invention relates to high pressure electric discharge lamps and, more particularly, to electrodes for such lamps, especially pressed-andsintered electrodes.

2. Description Of The Prior Art Pressed-and-sintered electrodes for high pressure electric discharge lamps are shown in U.S. Pat. Nos. 3,548,242 and 3,244,929 and In British Pat. Nos. 943,535 and 626,303. As pointed out in the references, it is sometimes desirable to locate the electron emissive material away from the surfaces on which the electric discharge terminates in normal operation.

For example, in U.S. Pat. No. 3,548,242, the emitter material is contained within a spring located in a slot at the rearward end of an electrode. However, in U.S. Pat.

No. 3,244,929, the emitter material is dispersed.

throughout the tungsten powder of which the electrode is composed. In British Pat. No. 943,535, also, the emitter material is dispersed throughout the body of the electrode; in addition, a second emitter material having a lower work function is disposed on the outer periphery of the electrode. In British Pat. No. 626,303, the emitter material is disposed in an axial hole in the electrode and a suitable plug covers the emitter material.

It is an object of this invention to provide an are discharge lamp electrode which is simpler and less costly to manufacture than prior art pressed and sintered electrodes, and which eliminates the need of close tolerance retaining means (e.g. springs or plugs) for the emitter material.

SUMMARY OF THE INVENTION A high pressure arc discharge lamp in accordance with this invention comprises a fused silica arc tube having press seals at each end thereof, molybdenum ribbons embedded within the press seals and pressedand-sintered electrodes within the arc tube adjacent the press seals.

The electrodes comprise powdered refractory metal, such as tungsten or tantalum, pressed and sintered into a tubular body with an axial hole throughout its length. One end of a refractory metal rod is securely fastened in the rearward portion of the axial hole and the other end of the rod is embedded within the press seal and connected to the molybdenum ribbon.

Disposed within the axial hole of the electrode and firmly pressed against the refractory metal rod and the walls of the axial hole is a pellet of emitter material. Such an electrode structure obviates the need of a spring or plug to retain the emitter material and offers the advantage thereover of lower starting voltage. The emitter pellet should be recessed into the hole more than about 25% of the length of the electrode body in order to be protected from direct arc stream attack.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevational view of a lamp in accordance with this invention.

FIG. 2 is an enlarged cross-sectional detail of an electrode of FIG. 1.

DESCRIPTION OF H B MENT i Asshown in the drawings,'a lampin accordance with this invention comprises a fused -silicatarc-tubefl having press seals 2 ateach end thereofrDisposed within arc tube 1 are pressed-and-sinteredelectrodes 3adjacent press seals 2. Electrode 3 is supported by and secured to, a refractory metal rod 4 extending into the rearward portion of an axial hole extending through electrode 3. The other end of rod 4 extends into press seal 2 where it is connected to an embedded molybdenium ribbon 5. Lead-in wires 6 extend externally from ribbons 5 and are the means for providing electrical power to the lamp.

Disposed within the axial hole of electrode 3 at about the center thereof is a pellet 7 of emitter material. Pellet 7 is secured within electrode 3 by having first been deposited therein as a cylinder and then being axially pressed against rod 4 sufiiciently to deform said cylinder and press it tightly against the walls of the axial hole.

An electrode for use in a 400 watt high pressure mercury vapor lamp in accordance with this invention had a length of 0.420 inches, a diameter of 0.161 inches and an axial hole diameter of 0.045 inches. Electrode 3 was prepared from a batch of tungsten powder, having an average particle size of 5.5 microns, thoroughly mixed with 1% wax and 0.2% zinc stearate. The material was pressed into a tubular body with axial hole therethrough at about 30,000 PSI and was then heated to l300C in wet dissociated ammonia to remove the wax binder and stearate lubricant and to give mechanical strength to the body.

Tungsten rod 4, 45 mils in diameter by 0.380 inches long, was inserted into the back of electrode 3 a distance of 0.125 inches and was sintered in place at l850C in vacuum for 1 hour.

Emissive pellet 7 was prepared from a batch consisting of 238 grams barium carbonate, 26 grams calcium carbonate, 136 grams thoria, 8 grams paraflin wax, 8 grams of tungsten powder and 0.4 grams calcium stearate and was pressed into a cylindrical shape, 39 mils diameter by 0.180 inches long. Pellet 7 was then inserted into the forward end of electrode 3, pushed against rod 4 and pressed to deform the pellet to the shape of the axial hole. The pressure used was in excess of 50,000 PSI.

The assembly was then heated at 1525C for 15 minutes in hydrogen to eliminate organic matter and to activate pellet 7 by converting the carbonates to oxides. The fmished lamp had one such electrode at each end of arc tube 1.

I claim:

1. A high pressure electric discharge lamp comprising: a fused silica arc tube having a press seal at each end thereof; molybdenum ribbons embedded in said press seals; a pair of electrodes disposed within said arc tube, each of said electrodes comprising a tubular body of pressed-and-twice-sintered refractory metal powder having an axial hole therethrough, said axial hole having been of uniform diameter after the first sintering; a refractory metal rod one end of which extends into said axial hole and is secured to the rearward portion of said ,body and the other end of which is connected to a mo- 3 a 4 after the first sint'eringand then second sintering the hole'bemee'n said pellet and said forward end being rod and tubular bodyat a higher temperature than the open first sintering; an emissive penet' disposed within the 2. The lamp vof claim 1 wherein said pellet is recessed central portion of said axial hole and secured therein by means of having been pressed against the walls of Said from the forward end of said body for a distance at least axial hole and said rod, said pellet being recessed from 25% of the length of Said bcdythe forward end of said body and the space in said axial 

1. A high pressure electric discharge lamp comprising: a fused silica arc tube having a press seal at each end thereof; molybdenum ribbons embedded in said press seals; a pair of electrodes disposed within said arc tube, each of said electrodes comprising a tubular body of pressed-and-twice-sintered refractory metal powder having an axial hole therethrough, said axial hole having been of uniform diameter after the first sintering; a refractory metal rod one end of which extends into said axial hole and is secured to the rearward portion of said body and the other end of which is connected to a molybdenum ribbon, said rod being secured in said rearward portion by having been inserted into said hole after the first sintering and then second sintering the rod and tubular body at a higher temperature than the first sintering; an emissive pellet disposed within the central portion of said axial hole and secured therein by means of having been pressed agaiNst the walls of said axial hole and said rod, said pellet being recessed from the forward end of said body and the space in said axial hole between said pellet and said forward end being open.
 2. The lamp of claim 1 wherein said pellet is recessed from the forward end of said body for a distance at least 25% of the length of said body. 